Measuring circuit



J. H. B-OLLMAN MEASURING CIRCUIT Oct-20, 1953 2 sheets-sheet 1 Filed Jan. 13;` 1951 /NVENrO/P J H. BOLL/VAN A TTORNEV Oct. 20,r 1953 H, BOLLMAN 2,656,506

I y MEASURING CIRCUIT Filed Jan. 13.,A 195,1 y y y 2 Sheets-Sheet 2 INVENTOP J. H. BOL/ MAN was@ *m- 7L@ A TTO/PNEV Patented Oct. 20, 1953 MEASURING CIRCUIT John H. Bollman, Fanwood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 13, 1951, Serial No. 205,949

9 Claims. (Cl. 324-57) This invention relates to the art of electrical measuring and in particular it relates to an apparatus for measuring harmonic levels generated in non-linear impedance elements.

One of the difficulties frequently encountered in the use of a non-linear impedance element is the development of harmonics by reason of modulation within the element. A major problem to be solved in eliminating them is the problem of accurately measuring their magnitude.

With certain non-linear elements, for example, a thermistor, modulation arises from a cyclic change of temperature of the thermistor element with the fundamental frequency current. This type of modulation is ordinarily kept 10W enough by proper design of the thermistor so that it does not degrade transmission. However, barrier layers or powdered thermistor material under the contact may cause excessive modulation. Where a thermistor is used for line regulating purposes in communication systems,`

it is essential that it introduce no appreciable modulation whatever. Loose contact modulation is substantially eliminated by thermally loading the element with the proper glass. The thermal expansion coeiiicient of the thermistor pellet must be carefully matched to the glass used to coat the thermistor in order to prevent crushing the thermistor. In the manufacture of these thermistors an occasional defective one is found due either to a defective thermistor rod or to improper heat treatment during the glass loading operation or contact application. Itis, therefore, necessary to check the modulation performance of these thermistors during manufacture. Measuring them at this time has two obvious advantages. It reduces the number of defective thermistors manufactured by locating and eliminating the cause of the defect and it prevents the shipment of defective thermistors to the field.

It is the object of this invention to provide Y a means for measuring the harmonic levels generated in a non-linear impedance element.

The foregoing object is achieved by this invention which comprises in combination an oscillator containing a bridge circuit adapted to stabilize the oscillation amplitude thereof, test terminals in said bridge for connecting the nonlinearimpedance to be measured, a pair of output terminals for said bridge, and a tuned meter circuit connected to said output terminals and responsive only Ato the harmonic whose level is to be measured.

The invention may be better understood Aby referring to the accompanying drawings in which:

Fig. 1 discloses a preferred embodiment of the invention adapted to measure a selected harmonic level of a non-linear impedance element; and

Figs. 2 to 6, inclusive, disclose a preferred means of setting up the apparatus of Fig. 1 to measure the selected harmonic.

For the purpose of illustrating this invention it is assumed that the non-linear element to be measured is that of a thermistor although other kinds can be similarly measured. The thermistor is designated by reference character RT in Fig. 1. This element is connected to the test terminals TT of 'bridge l by means of a pair of leads having van inherent capacitance CT. Bridge l is essentially a resistance bridge and comprises resistors R1, R2, a iixed known resistance R and the thermistor resistanceR'r. The inherent capacitance CT of the leads connecting the thermistor to the bridge terminal is balanced out by means of a capacitor C1. The input terminals of the bridge are connected to the output terminals of amplier 5 through a transformer AIl, while the bridge output terminals are connected to the input terminals of amplifier 5 by way of an obvious path. The same output terminals of the bridge are also connected to a measuring circuit including ampliers 6 and 1, amplifier-rectifier 8 and meter M. The output terminals of amplifier 5 are connected by means to plug PI and jack J I to a tuned network 3 `which is connected to the primary of transformer 4. It will thus be observed that bridge I, amplier 5 and tuned circuit 3 comprise a conventional rbridge-stabilized oscillator of the type shown in Fig. 3 of United States Patent 2,066,333 grantedJanuary 5, 1937, to R. S. Caruthers. The frequency of oscillations is determined primarily by the tuned network 3 while the amplitude level is determined largely by the network of bridge i and is stabilized by the nonlinear characteristic of the thermistor RT. It is preferable that tuned circuit 3 be external to the bridge as shown to increase the discrimination of the meter circuit as hereinafter more fully explained.

The voltage across the thermistor RT is measured by a vacuum tube voltmeter connected across resistor R, this measurement being made by closing switch S3.

The measuring circuit comprising amplifiers 6 and 1, amplier-rectier 8 and meter M is for the purpose of measuring a selected harmonic generated in the thermistor RT. The tuned circuit comprising inductor L and capacitor C2 is tuned to resonance with the harmonic which it is desired to measure. Amplifier 6 is preferably a broad-band distortionless amplifier. Amplifier 1 is preferably capable of being tuned to the selected harmonic and should have a variable gain control. -While this amplifier may be of 'any conventional type it is preferable that it be in the form of a frequency calibrated heterodyne detector which provides both the tuning and the variable gain function. AmpliBr-erentifier 8 may be of any conventional type which receives an alternating voltage fat-its input `terminals and delivers a rectified nect Voltage t 5 meter M. Meter M is preferably calibrated in decibels referred to 1 milliwatt tdbm).

The use of a bridge-stabilized oscillator as a component part of this measuring system has the unique advantage of bringing every ther- `mister to "be measuredto vsubstantially the same predetermined, known mean resistance devel. This `lis done `ver-y Aquickly and :automatically thereby'facilitating rapid measurement. Another Aoutstanding and very vunique property of such a bridge network is that the harmonics generated in the ithermistor lelement are only slightly attenuated 'by -the 'bridge network itself whereas the fundamental E'frequency and all Fharrncnics Eand 'noise 4componentssupplied toA A"the bridge by amplifier Aare highly attenuated "by the bridge. 'This gives considerable frequency discrimina- 'tion to lthe -measuringfcircuit in addition-to that provided "by its tuning. 2in-arrangement of the type shown inFig 1 iis-easily designed lso 1that the residual harmonics applied to the 'input terminalso'f bridge l Aas Vwell as the noise fcomponentsare well below the level of 'the harmonics generated fin the 4ther-mister itself. The afore- Vmentioned `frequency discrimination @arises from Ythe fact that the 'input terminals'of the 'bridge iareconjugatewith-the output terminals whereas .the -ithernistor terminals iare vnoiwconjugate with fthe 'bridge output terminals. lf-I'hermeter lM "in r*the tuned detectorampli'fier `just described responds substantially ronly to lthe selected harvmonic.

:In order'fto efacilitate asetting -up f' the Aafpllalaiius zof Fig. f1 @to properly Lmeasure-a selected'harmonic,

switches iSi, .S2 and A:jacksJL 'J2, 'J3 and J4 are `provided. Switch SI yTis providedto short-circuit .capacitor ,C2 x.thereby eliminating the vtuning efvfeet of this'capacitorwithinductor'L. Switch S2 is-provided asa convenient means for disconnectringtbalancing capacitor Cl duringcertain phases of the set-up :procedure 'The several jacks re- Eerredto'provide convenient means for #connecting ithe apparatus together in diierentcombinations during the set-upiproce'dure. "The-preferred procedure isbest understoodby referring to Figs. 2 to 6, inclusive.

AIrl-.therst;step o'fthis 'procedure it -is desired to .so-adjust the l.gain control of ampliner 1 as to -provide lno'net` gain be'tween "its "input #and output terminals. ',Tlnsiis :accomplished 'rby means of the arrangement shown :in Figs. '21and 53. Switch SI .is .closedtoneliminate :theztuning effect of -capacitor C2 and inductor L. Switch S2is'left open and .a-iixedfresistorgRfr'is connected-to test terminals TT bymeans of .leads Vhavingan "inherent ycapaci- 1 .tance YGir. Itis .preferred that this capacitance be substantially the same asfcapacitance CT of theleads connecting thethermistors to be meas- .ured fand .that Yresistance :RT 'tbe Ysubstantially @ella-1 .t0 the :desired thermistor resistance. Plug Pl is removed from jack J I and a variable voltage, variable frequency oscillator 9 is connected to this jack. Amplifiers 6 and 'I are removed from the measuring circuit by substituting plug P4 for plug P2 in jack J2. Oscillator 9 is then adjusted in frequency until a maximum deiiection is noted in meter M. This oscillator is thus tuned to the i'requencyoffnetwork 3 .whichgis rtheifundamental frequency vci the bridge-stabilized oscillator of Fig. l. Oscillator 9 is also adjusted in voltage level so as to give a convenient deiiection on meter Mzwinch may be substantially half full scale deflection. The magnitude of this deflection is noted.

Plug P'isithenremoved from jack J 2 and reins'erted in jack U11. Plug P3, as shown in Fig. 3, is insertediniackzfof Fig. 2. This change in connections inserts amplifier 'l in the measuring circuit, y'the circuit otherwise remaining identical with that of Fig. 2. The tuning of amplifier 'I is nowadjustedzuntil VAa maximum Iresponse is observed lonarne'terilvl therebygalso'ztuning'thisamplizlier -torthe frequency .of :network :3. AThe vgainof amplifier 1 is 4adjusted `until meter -M gives .the fsamefdefiection .as it did vin Fig. 2. Amplier 1, .when .so adjusted, `will introduceno gain or loss Vvin "the transmission 'path :of .the Ameasuring '.cir-

fcuit.

In thenextfstepthe apparatus of Fig.'3 is'modified as showninFig. 4. .modification comprises ronly :thead'dition :of .a Aresistor .R :and the connection fof :balancing capacitor zCl lby `the :c'lo- -sure nf switchV S2. The zpurpose lvof l.this set-:up nis to :provide a :complete vbridge balance VAof network l for 'the fundamental frequency vof netiwork ;3. In fact, :this .adjustment substantially balances :the :bridge for .frequencies 'byA reason `of '.the .symmetry .ofithe ibridgetnetivork. The -.balaancing '.is .accomplished aby @merely adjusting re- Y:sistorlftl :and capacitorigl. .Itspreferablezthat thefresistance of :resistor 5R :bear 'a decimal :m111- xtiple :relationship Twith xrespect ato resistor Rrr' '.fso :that :the `voltage :measured 'across :resistorRin ia subsequent :operation 'will :bear a :simple l.decimal .multipleirelationship with .the voltagesacross ,the thermistor. .'In zone '.fembodiment this `resistance twas made one-tenth ,the value :of fthe ldesired thermstorasesistance.

.Referring .now :to 1155s.v 5it iwill be; noted :that'the apparatus-.has been againmodined, this :time re- -fstoringthe :bridgeestablized .oscillator circuit .of Eig. 1 fbyrfreconnecting ampliner 5to the-network 3. Amplifier ft.' has also `:been finsertedinthe cir cuits-iso that the measuring :circuit is rnow like lFig. r1.. iInrFgL-5ias `wellasin Fig. 'T1 the "various .,lectedffor measurement. :For thirdordermodu- `.lation this will -be the third-harmonic sandthis harmonic :may be assumedas a specific .example illustrating .thetinuention. .Notwithstanding the `.tuning feiect of Nnetwork a considerable Y`third harmonic level will be applied to the 4input ter- :minalsfof'the bridge network I'which, icy-reason of the fact that this bridge is strongly unbalanced,- will also appear at the output Vterminals with very little attenuation.- The tuning of amplifier 'l is changed to correspond to the selected harmonic, in this case the third harmonic, this condition being noted by a maximum deection of meter M. Switch Si is opened and capacitor C2is adjusted for a maximum deflection of meter M, thereby also tuning inductor L to the third harmonic.

There remains now only a Ycalibration step which is accomplished by applying a known third harmonic level to the test terminals TT and noting the level measurement of meter M. 'Ihe correction factor obtained from this calibration step is the difference between these two readings. This is more easily understood by referring to Fig. 6.

In Fig. 6 it will be noted that the fixed resistor RT has been reconnected to the test terminal TT thereby substantially balancing the bridge l for all frequencies and in particular the fundamental frequency. Amplifier 5 should be deenergized since it provides a feedback path which will modify the apparent impedance of the bridge and hence lead to a false calibration. The variable frequency oscillator 9 and a calibrated attenuator l of conventional construction is connected by means of conductors l I to the test terminals TT. The input level of oscillator 9 to the attenuator I0 is adjusted to zero decibel referred to l milliwatt by any convenient laboratory means not shown. Since attenuator l0 is calibrated, it follows that the output level at the output terminals of attenuator lil will be at a level below the zero reference level bythe amount indicated by this attenuator. The frequency of oscillator 9 is adjusted to give a maximum response in meter M thereby adjusting the frequency of this oscillator to the selected third harmonic. Attenuator I 0 is then adjusted to give a convenient reading on meter M. The difference between the loss reading of attenuator l0 and the reading of meter M is the correction factor g which represents the difference between the gain of amplifier 6 and the loss out of the bridge l.

Lines Il and the fixed resistor RT' are then removed from the test terminals TT and amplifier reenergized. The apparatus is then ready for measuring thermistors or other non-linear elements for generation of the selected third harmonic. The resulting circuit structure is identical with that shown in Fig. 1. To make these measurements a thermistor R'r is connected to the test terminals as shown in Fig. 1. Switch S2 is left closed. If the resistance of resistor R is to be one-tenth of that of the non-linear resistor R'r the balancing set-up of Fig. 4 will have adjusted resistor Rl to a resistance equal to onetenth of that of resistor R2. In a specic embodiment where the non-linear resistance is that of a thermistor and Rl is made 500 ohms, R2 5,000 ohms and resistor R 50 ohms, the bridgestabilized oscillator will bring the thermistor resistance to closely approximate 500 ohms. How closely this resistance approaches 500 ohms will be determined largely by the gain of amplifier 5 which is preferably made relatively high. A 65- decibel gain has been found satisfactory. The level of the selected harmonic is then obtained by simply adding the reading of meter M to the correction factor g previously obtained in the calibration- If the meter reading is denoted G the harmonic level may be expressed mathematically as follows:

where L represents the harmonic level generated in the'non-linear element.

If it-is desired to measure the voltage across the thermistor at which this harmonic level was generated, switch S3 is closed thereby measuring the voltage across resistor R. Since the ratio of resistor R to the non-linear resistor is known the voltage across the'non-linear resistance is easily determined. For the example given, the voltage across the resistance RT is ten times that of the reading ofthe vacuum tube voltmeter.

-The power loss in resistor RT is obtained by simply taking 'the ratio of the square of the volt-'- age across this resistance to its resistance. For the specific example here assumed it is expressed as follows: v

where P is the power loss in the units indicated and V is the voltage across resistor R.

A particular method of setting up and calibrating the apparatus has been described in connection with Figs. 2 to 6, inclusive. It is obvious to any one skilled in the art that other methods may also be employed. It is only essential that the bridge-stabilized oscillator be so arranged as to bring the non-linear impedance element to a known mean resistance level and that the detector-amplier circuit betuned to the selected harmonic which it is desired to measure. The calibration should be such as to provide' a, known relationship between the reading of the measuring instrument and the harmonic level across the terminals `of the non-linear impedance element. Itwill also be obvious to those skilled in this art that the particular form of stabilized oscillator specically'disclosed to illustrate the invention need not be employed but other bridge-stabilized oscillators capable of performing the same function may be substituted.

What is claimed is:

1. A measuring apparatus for measuring a harmonic level generated in a non-linear impedance comprising an oscillator having a feedback path and including a bridge circuit conn nected into said feedback path to stabilize the oscillation amplitude thereof, test terminals in said bridge for connecting the non-linear impedance to be measured, a pair of output terminals for said bridge, and a tuned meter circuit connected to said output terminals, said meter circuit including a tuned circuit tuned to a selected harmonic above the fundamental frequency of said oscillator whereby the meter is responsive only to said selected harmonic whose level is to be measured.

2. A measuring apparatus for measuring a harmonic level generated in a non-linear impedance comprising an oscillator having a feedback path and including a bridge circuit connected into said feedback path to stabilize the oscillation amplitude thereof, said bridge circuit comprising three substantially pure resistive arms and a fourth arm including test terminals to which said non-linear impedance may be connected, a pair of output terminals for said bridge, and a tuned meter circuit connected to said output terminals, said meter circuit including a tuned circuit tuned to a selected harmonic above the fundamental frequency o-f said oscillator whereby the meter is responsive only to said selected harmonic whose level is to be measured.

3. A measuring apparatus for measuring a.

harmonic vseeiterated -ina znpnfflinear fimpedance comprising an oseillatqr of the :bridgestabilized type, a bridge circuit in .said oscillator, said bridee Y.circuit comprising three :substantially nur e lresistive arms and a 'fourth arm including test` terminals to .which :sa-id non-.linear irnuedanee :may be connected, whereby the oscillation 4level is determined by `the ,resistaneefcf said V,non-'linear impedance, a pair of .Output terminals fior said bridge, land a tunedmeter circuit cork nected to saidoutput-terminals, said metercircuit including .La .etui-led circuit tuned to a selected harmerne .above the :fundamental .frequency of Vsaid oscillator .whereby `the .meter is .responsive @my to said .selected .harmonic whose levelis te be measured.

4. The combination of .claim 1 wherein said meter circuit Vincludes a tuned "variable gain amplifier capable of being tuned to the harmonic WhoseLlevel-is to be measured.

.5. Ehe combinatien of claim 2 wherein said meter circuit includes .a .tuned variable gain amplifier vcapable of being tuned to the harmonic whoserlevelfis .to be measured.

6. '131e combination of claim 3 .wherein said meter .circuit includes a tuned variable gain 4amplifier capable of being tuned to the harmonic .whose `level is to be measured.

7.. The combination of .claim 1 wherein .said bridge comprises two substantially pure resistive arms, .a third l,arm .containing an impedance in- .cludingmeans 'for .adjusting its .phase angle to .equal .that `of the non-linear impedance `atsubstantial bridge balanceand wherein said test terminalscomprise a fourth-arm.

,8. .A .measuring apparatus for measuring .a harmonic level generated in a knon-linear impedance comprising `Aanoscillatcn `having .a feed- .baQk Lnath ,and including a .bridge circuit ,con-

neoiediinto .said ,feedback path to istabilize oscillation :amplitude thereof, a fundamental fre queues/:determining means included said .Oscillator v.exterior @ef-.said bridge test-terminals ,insaid bridge :ieri-,connecting the `non-linear impedance to bemeasured, a pair of output terminals ,conneeteddreetly-io saidbridge. .and a tuned :meter circuit connected 6.04 Said output terminals, said meter circuit :including a tuned circuit tuned to a ;e1eeted-liarmonic labove 4the fundamental ;f;re guency .of said oscillator whereby the meter is respensire .only tesaid: selected `harmonic whose Ieuelfisito befmeasured.

PA measuring apparatus fer measuring n hamer-ric A.lei/rel generated .in a Ynon-linear imuedauee-.eemur-isine .a bridge-.stabilized oscillator. the bridge whereof having conjugate 'input and eutputrterminais..afundamentalfrequeneyzdetermining .means exnirier of the bridge .and foon- 'nied :to -saidrinput fterminalsthereofv, test terminels .in :said bridge for connecting the nonlinear Iirnpedance to :be measured, and a tuned meter circuit connected to said output terminals., saidmeter .circuit including a tuned circuit tuned to .a vvselected harmonic above the fundamental .frequency of said' oscillator whereby the meter is .responsiveonly to said selected harmonic Whose level is tobe. measured. 

